The primary function of the suit is to regulate Black Lightning's power
Jefferson Pierce has the ability to draw in electrical energy from his surroundings and release it through his hands. Although he can do this without using special equipment ("The Resurrection", 1x01; "Black Jesus", 1x04), the process is dangerous for him. As Black Lightning, his suit protects him from the harmful effects of his own power:
"That suit of yours is what keeps you from blowing out your nervous system when you use your power."
— Peter Gambi, "Three Sevens: The Book of Thunder", 1x06
The suit also gives Jefferson better control over his natural abilities, such as by increasing the accuracy of his blasts ("The Book of Burial", 1x03) and by allowing him to use his hands as thrusters for flight ("And Then the Devil Brought the Plague: The Book of Green Light", 1x05).
In its primary function, the suit does not require a helmet.
Black Lightning is a symbol to the people
"Do you remember why you became Black Lightning? You wanted to give the people hope."
— Peter Gambi, season recap for "Three Sevens: The Book of Thunder", 1x06
As a black hero, Black Lightning inspires black people to live just lives while reminding them that they have a powerful person supporting them. To do this, he needs for people to see that he is black.
The suit covers most of Jefferson's body, except for his head and some of his fingers. To conceal his civillian identity, he already wears a mask and uses his power to blur his face and voice; if he also wore a helmet, he would lose his identity as an unambiguously black hero.
Helmets symbolize repression
Heroes sometimes chose not to use abilities that have unwelcome associations. For example, Batman has forsaken guns — even though they would add to his arsenal of crime-fighting tools — because of his childhood experience.
Helmets are practical, but they have an image problem: members of the riot squad wear them.1 Given Black Lightning's interest in social justice, he might not be comfortable — or believe that he could be an effective symbol — having the same look.
1 Google Images shows press photos of encounters between riot police (wearing helmets with full-face shields) and black protesters. I have not shown these photos here because of potential copyright issues, but they are easy to find.